1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device in which a surface source electrode of a MOSFET (insulated gate field-effect transistor) is electrically connected to a lead frame via a strap member. For example, the present invention is used in a trench gate type power MOSFET.
2. Description of the Related Art
FIG. 5 is a plan view schematically showing a portion of a conventional trench gate type power MOSFET using a wire bonding structure. FIG. 6 is a sectional view schematically showing a structure taken along a line VI—VI in FIG. 5.
This power MOSFET is comprised of a plurality of MOSFET cells, and includes a surface source electrode (Source-AL) 13 connecting to source diffusion layers 4 and a surface gate electrode (Gate-AL) 14 connecting to a gate polysilicon wiring layer 7a. The MOSFET cells are formed in a semiconductor chip 40 which is mounted on a lead frame (not shown). A plurality of wires (Source-Wire) 41 are connected by bonding so as to electrically connect the surface source electrode 13 to the lead frame. A wire (Gate-Wire) 15 is connected by bonding so as to electrically connect the surface gate electrode 14 to a lead portion of the lead frame.
As each of the surface source electrode 13 and surface gate electrode 14, an Al electrode mainly composed of Al is used. A portion of the gate polysilicon wiring layer 7a is formed into a planar stripe in the array of the source diffusion layers 4. To reduce the internal resistance (to be referred to as rg hereinafter) of the gate, a gate Al (Gate-Al) wiring layer 42 mainly composed of Al is formed in contact with the upper surface of the gate polysilicon wiring layer 7a. The gate Al wiring layer 42 and the surface source electrode 13 are two-dimensionally interdigitated.
Recently, it is strongly demanded to reduce the ON resistance of a power MOSFET. As a package for achieving this demand, a strap member structure which connects the surface source electrode by a strap member is attracting attention. A representative example is Jpn. Pat. Appln. KOKAI Publication No. 2000-114445.
The conventional power MOSFET strap member structure connects a Cu strap member to the surface source electrode via an adhesive such as Ag paste.
Unfortunately, in reliability tests such as a temperature cycle test for a power MOSFET, this strap member structure shortens the life of the power MOSFET because the thermal expansion coefficients of an Al electrode on the chip surface, the Ag paste, the Cu strap member, and a Cu lead frame are different.
To solve this problem, a method of connecting a strap member by using ultrasonic waves (to be referred to as US hereinafter) has been proposed.
FIG. 7 is a plan view schematically showing a portion of a conventional power MOSFET using a strap member. FIG. 8 is a sectional view taken along a line VIII—VIII in FIG. 7. In FIGS. 7 and 8, the same reference numerals as in FIGS. 5 and 6 denote the same parts.
The reliability of this power MOSFET strap member is greatly improved by connecting a Cu or Al strap member (Source-Strap) 16 onto a surface source electrode 13 by US.
In this structure, the strap member is connected onto the surface source electrode 13. Therefore, if a gate Al wiring layer 42 exists as in the prior art shown in FIGS. 5 and 6, the gate Al wiring layer 42 is broken and short-circuited to the surface source electrode 13 when the strap member is connected by US near the gate Al wiring layer 42. This often causes a short circuit between a gate G and source S.
Accordingly, the gate Al wiring layer is omitted as shown in FIG. 7, so the gate Al wiring layer 42 and the surface source electrode 13 are not two-dimensionally interdigitated.
The existence, however, of the gate Al wiring layer 42 has a large effect on the internal resistance rg of the gate. If this gate Al wiring layer is omitted as shown in FIG. 7, rg will be increased about twice (approximately 3Ω) as compared with about 1.5Ω when the gate Al wiring layer 42 exists as in the prior art. This rg rise lowers the conversion efficiency when the power MOSFET is used for synchronous rectification. Therefore, this power MOSFET cannot be used for synchronous rectification.
In the conventional power MOSFET as described above, no gate Al wiring layer can be formed when a strap member is connected onto the surface source electrode by US in order to improve the reliability. This will result in the increase of the internal resistance rg of the gate.